The present invention relates to ferritic stainless steel plates and foils with improved resistance to oxidation and a process for their manufacture.
Recently, an Fe-Cr-Al base alloy has been widely used as a superior heat-resistant material in the manufacture of heating stoves and motor vehicle exhaust gas converters. In particular, a stainless steel foil having improved resistance to impact has been used in place of conventional ceramics as a catalyst carrier for use in exhaust gas converters of motor vehicles. As service conditions are becoming more and more severe, further improvement in heat resistant properties is required.
It has been known that heat resistance of Fe-Cr-Al alloys can be markedly improved by the addition of Y. However, it is also true that the addition of Y degrades toughness of a hot-rolled plate of an Fe-Cr-Al base alloy so markedly that the occurrence of troubles such as cracking and rupture of steel is inevitable during uncoiling or cold rolling.
In order to avoid such a degradation in toughness, Japanese Patent Unexamined Laid-Open specification No. 60-228616/1985 proposes to rapidly cool a steel plate with a reduced content of C and N at a cooling rate of 10.degree. C./sec or larger and to coil it at a temperature of 450.degree. C. or lower. However, even when such a process is applied to an Fe-Cr-Al base alloy containing Y, a satisfactory level of toughness cannot be attained.
Thus, the prevailing method at present comprises carrying out warm rolling after heating a hot-rolled plate to 100.degree.-400.degree. C., and reductions in working efficiency and yield are inevitable, resulting in an increase in manufacturing costs.
When the above type of alloy is used to manufacture an exhaust gas converter for automobiles, an extremely thin foil having a thickness of 50 micrometers or smaller after rolling is assembled to form a honeycomb structure. Since the thickness of the foil compared with that of a ceramics honeycomb is very small, the resistance to flow through the structure is reduced due to a reduction in a sectional area of the honeycomb structure, resulting in an improvement in engine performance.
It is necessary to heat a catalyst in start-up procedures. The start-up procedures require a substantial length of time and the catalyst does not work until it is heated to a given temperature. On the other hand, the thinner the thickness of the foil, the smaller the heat content of the honeycomb structure. It is possible to shorten the length of time to reach the given temperature by making the foil thinner.
In contrast, the resistance to oxidation is markedly degraded as the thickness of a foil decreases. The Al content of a foil also has an important influence on the oxidation resistance. The larger the Al content, the more the oxidation resistance is improved. However, when the content of Al is increased beyond a certain point, the producibility and workability of the steel plate are impaired to make it difficult to mass produce foils in an economical way.